1. Field of the Invention
The present invention relates to a novel coupled polymer, and more particularly to a process for preparing the novel coupled polymer by using a particular coupling agent.
2. Description of the Prior Art
Living alkali metal-terminated polymers have been produced by polymerizing conjugated dienes or polymerizing conjugated dienes and monovinyl arenes in the presence of an organic alkali metal compound as an initiator. Such a living polymer is then reacted with a coupling agent to form a linear or star polymer. The coupling center of the coupling agent thereby becomes a nucleus for such a polymer. Such coupling is relatively efficient and stable and has been used for the production of polybutadiene rubbers, styrene-butadiene rubbers, and thermoplastic elastomers, such as styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), styrene-ethylene/butene-styrene (SEBS), and styrene-ethylene/propylene-styrene (SEPS).
The conventional coupling agents used for coupling have some problems. British Patent 1014999 uses a dihalogenated hydrocarbon as a coupling agent to prepare an ABCBA coupled block copolymer, which is formed by the reaction of the coupling agent and copolymeric carbanion AB.sup.-. Such a coupling agent has a problem that by-products (such as chloride of the alkali metal) are generated, which will not only erode the pipes, but also remain in the rubber and react with the anti-aging agent to yellow the rubber; thus, the rubber obtained is opaque. In addition, such a coupling agent has been banned by many countries because of its toxicity.
British Patent 1103939 uses carbon dioxide, carbon disulfide, or carbonyl sulfide as the coupling agent to form an ABCBA coupled block copolymer. Such a coupling agent has low efficiency. Since carbon dioxide is a gas, it is very difficult to control its dissolved concentration. In addition, sulfide by-products will be generated after coupling, contaminating the polymer and causing the problem of wastewater.
U.S. Pat. No. 3,668,279 uses maleic anhydride or dimethylene terephihalate as the coupling agent. The problems are that the coupling efficiency is low, the coupled polymer solution is yellow, and the alcohol by-products will poison the organic alkali metal initiator.
U.S. Pat. Nos. 3,244,664, 3,692,874, 3,880,954, 3,725,369 use silicon halide, siloxane, silyl amine, or silyl sulfide as the coupling agent. Most of the polymers obtained are star polymers and the coupling efficiency (coupling ratio) is high. However, by-products including alcohols, organic amines, and thiols will be generated, which will remain in the recovered solvent, thus poisoning the organic alkali metal initiator. Moreover, the polymer obtained will have the bad odor of amines or thiols.
U.S. Pat. No. 4,039,633 uses 1,3,5-benzenetricarboxylic acid trihalogen as the coupling agent. The result is that a large amount of uncoupled polymers and coupled polymers with an uncertain coupling number are formed. Since the coupled polymer contains a carboxyl group, it easily become yellow. Moreover, the alkali metal halide will erode the pipes and make the product opaque.
U.S. Pat. No. 3,468,972 uses polyepoxides, polyisocyanates, polyimines, polyaldehydes, polyketones, polyanhydrides, polyesters, or polyhalides as the coupling agent. It is very difficult to control the coupling number, and the star polymer obtained is the mixture of coupled polymers with various coupling numbers. Moreover, the coupling agent has a high molecular weight, thus it is difficult to dissolve in hydrocarbon compounds. The coupling efficiency is lower than 60%, the product has undesired color and odor, and it is often accompanied by by-products of alcohols or halides.
U.S. Pat. No. 3,281,383 uses styrene-acid anhydride copolymer, epoxy liquid polybutadiene, or polyphenyl isocyanate as the coupling agent. The disadvantages are similar to those mentioned in U.S. Pat. No. 3,468,972.
U.S. Pat. No. 4,107,236uses diesters or silicon halides as the coupling agent. The disadvantages are similar to those mentioned in U.S. Pat. Nos. 3,244,664 and 3,668,279.
U.S. Pat. No. 3,985,8330 uses m-divinylbenzene as the coupling agent. The coupling efficiency is not high, and the range of the arm number is undesirably broad. That is, the arm number can be 2 (linear polymer) or 12 (star polymer). In addition, the commercially available divinylbenzene is usually a mixture of para, ortho, and meta; therefore, the applicability of m-divinylbenzene as the coupling agent is restricted.
U.S. Pat. No. 4,049,753 uses an anhydride of a monocarboxylic acid as the coupling agent to prepare a polymer having ether or alcohol groups on its coupling center. The coupling efficiency is not high and the polymer (rubber) obtained will turn yellow because of the presence of anhydride and carboxylic acid. Moreover, the rubber has inferior transparency.